1. Field of the Invention
The present invention relates to a method of treating a subterranean formation. More specifically, the invention is a method for selectively acidizing a primarily oil producing zone in a formation comprised of a primarily hydrocarbon producing zone and a primarily water producing zone.
2. Description of the Prior Art
Certain oil producing wells are formed or completed in formations which contain both oil producing zones and water producing zones. Unless measures are taken to change or effect the production rates from each of these zones, the particular reservoir characteristics (i.e., water cut increase and trend, total liquid inflow rate increase and trend, oil and water cumulative production cross-plots, and down-hole inflow profile surveys) will dictate the amount of water and oil produced from the well.
Acidizing is an art that applies a reactive fluid, such as a matrix acidizing composition, including solvents and additives, to contact and dissolve wellbore damages. Zone selectivity may be achieved by using a mechanical method, such as coiled tubing with packers, or by using a fluid or an additive which diverts the reactive fluid from zone to zone, as needed. Conventionally, target treatment zones are isolated by mechanical methods and tools such as bridge plugs, inflatable packers or temporary chemical blocking systems or the combinations of thereof depending on wellbore completion and conditions. These methods, if applicable, provide zonal isolation only at the wellbore.
Alternatively, a water zone or layer may be shut-off by injecting a delayed gelling fluid or a delayed precipitating fluid into that layer. To ensure proper and accurate zone selection, the location and source of water must be identified and a mechanical or chemical zone isolation method then applied.
The use of viscoelastic surfactant gels have also been shown to divert the acid or the reactive fluid to preferentially enter a specific zone or layer and may even avoid acidizing water zones. Recently, viscoelastic surfactant gels has been developed not only for fracture stimulation but also for acidizing. A good example is the use of a viscoelastic surfactant to formulate a proppant carrying fracturing fluid for hydraulic fracturing. The use of viscoelastic surfactant gels or the like, either foamed or unfoamed provides more effective temporary isolation since these fluids are capable of penetrating deep into the rock formation.